This invention pertains to the electronic control of viscous fan drives such as those used in automotive cooling systems. More particularly, this invention relates to the control of viscous fan drives to manage slip power loss.
Controllable viscous fluid clutches are being used in automotive engine cooling fan assemblies and additional applications are being developed. Control of torque transfer in the fan drive clutch means that, for example, fan speed can be controlled for more efficient vehicle operation and better cooling. The design and mode of operation of these controllable clutches take different forms. In general, the power input member of the clutch is driven by the engine or by an auxiliary power source. The output member drives the fan to draw cooling air through the heat exchange elements of the radiator, oil cooler and air conditioner condenser. A viscous fluid is used in these clutches to transfer torque from the power input member to the output member. In the operation of these devices, the power input member often rotates at a higher speed than the output member. The difference in their speeds is called slip or slippage. Slip represents a power loss in the fan drive clutch.
In one type of clutch, the electroviscous (EV) clutch, the control of torque transfer is achieved by controlling the amount of fluid, often a silicone oil, permitted to flow into the working chamber between the clutch members. In another type of controllable clutch a magnetorheological (MR) fluid is used and the control of torque transfer is managed by controlling the strength of a magnetic field in the fluid gap between the clutch input and output members. Clutches using MR fluids are particularly responsive to electronic control.
U.S. Pat. Nos. 5,960,918; 6,032,772; 6,102,177; and 6,173,823, each entitled xe2x80x9cViscous Clutch Assemblyxe2x80x9d describe clutches using magnetorheological fluids as the viscous medium between the power input and output members of the clutch. U.S. Pat. Nos. 5,555,963 entitled xe2x80x9cElectromagnetically Controlled Hydraulic Clutchxe2x80x9d and 5,722,523 entitled xe2x80x9cFluid Clutchxe2x80x9d describe electroviscous clutches. The disclosures of these patents are incorporated by reference into this specification to illustrate the kinds of viscous fluid clutch devices that can be controlled by the practices of this invention.
U.S. Pat. Nos. 5,667,715 entitled xe2x80x9cMagnetorheological Fluidxe2x80x9d and 6,149,832 entitled xe2x80x9cStabilized Magnetorheological Fluid Compositionsxe2x80x9d describe viscous fluids for use in MR fluid clutches. As disclosed, they often contain finely divided iron particles suspended in a non-polar liquid. The suspended ferromagnetic particles have low magnetic coercivity. The fluids are formulated to resist particle separation even under high separation force applications and they typically function as Bingham fluids. In an ambient gravitational field, but in the absence of a magnetic field, they display a shear stress that increases generally linearly as the shear rate on the fluid is increased. When magnetorheological Bingham fluids are subjected to a magnetic field, the shear stress vs. shear rate relationship is increased so that substantially more shear stress is required to commence shear of the fluid. Thus, the shear stress and effective viscosity of the MR fluid can be instantly varied by controlled application of a magnetic field in the volume occupied by the fluid. This characteristic is very useful in controlling the transfer of torque between a rotor and stator in a viscous fluid clutch assembly.
The cooling fan is driven whenever the temperature of the engine coolant, the engine lubricating oil or the automatic transmission fluid reaches a predetermined temperature, or when the air conditioner high side pressure reaches a specified level. Temperature and pressure sensors are appropriately located for this purpose and their respective signals continually monitored by a computer based control module for the engine or powertrain. When cooling is required, the controller commands a voltage to be applied to the coil assembly of the clutch assembly. By control of the magnitude of the applied voltage, a suitable electromagnetic field is established in the clutch gap occupied by the fluid. The responding variation in the effective viscosity of the MR fluid is the basis for controlling fan speed.
Variation in the viscosity of the MR fluid permits slippage between the clutch rotor and stator members and the fan speed is controlled to efficiently and effectively cool the fluids used in operation and cooling of the car or truck. In fact, usage of controllable viscous fluid cooling fan drives, especially MR fluid fan drives, provide vehicle fuel economy improvement, noise reduction, powertrain cooling improvement and cost reduction. However, the slippage in the fan drive clutch does result in slip power generation and loss, which translates into clutch temperature increase. It is desirable to limit, if possible, the amount of slip power for several reasons, fan drive durability being one of them.
Accordingly, it is an object of this invention to provide a method of controlling the operation of controllable viscous fluid fan clutches to avoid high slip power. It is another object of this invention to provide such a method that can be conducted by the engine control module or equivalent fan control computer.
This invention is a method for controlling the operation of an electronically controlled viscous fan drive to avoid excessive slip power loss. In accordance with a preferred embodiment of the invention, the invention is applied in the control of a MR fluid cooling fan clutch for an automotive vehicle using an engine driven fan drive such as a pickup truck. But the invention is applicable to other controllable viscous fluid clutches such as EV clutches.
In the case of a vehicle, an engine driven cooling fan is used to draw ambient air through one or more heat exchangers to extract heat from fluids such as engine coolant, engine oil, automatic transmission fluid and air conditioner refrigerant. The cooling fan assembly includes a viscous fluid clutch in which the rotor is driven by the engine. The viscous fluid, preferably a MR fluid, serves to control the operation and speed of the clutch stator to which the fan is connected. A given fan design has known air moving capabilities depending upon its rotational speed resulting from the torque applied to drive it. In a MR fluid clutch an electromagnetic field generating coil is used to control the effective viscosity of the MR fluid and thus the torque driving the fan.
Many functions of modern automotive engines (and transmissions) are controlled electronically with an engine or powertrain control module. The heart of the module is a suitably programmed computer with its supporting clock, input/output devices, memories and the like. Such a control module is used to control cooling fan speed. During its processing cycles repeated many times per second the module monitors several vehicle operating parameters including engine coolant and oil temperatures and refrigerant pressure. It interprets these signals in accordance with predetermined data in its memory to regulate the speed of the cooling fan.
The difference between clutch input speed and fan speed translates into slip power loss which tends to increase the temperature of the fan drive assembly. This invention provides a process for control of fan clutch operation to maintain necessary cooling without experiencing excessive slip power loss and therefore excessive temperature rise which can lead to premature failure of the fan drive.
Slip power is determined for a given fan drive assembly as a function of the fan torque constant, fan speed and slip speed. Based on this analysis an excess slip power region is identified as a function of fan speed and input speed. In the control of fan speed by the controller computer, cooling is accomplished while managing fan speed to avoid, where possible, the excessive slip power region. In the MR fluid clutch, fan speed is controlled by variation of the magnetic field acting on the MR fluid to vary its shear stress and effective viscosity. In other controllable viscous fluid clutches, a different fluid parameter may be used to control fan speed and slip power. For example, in the EV clutch, the volume of the fluid in the working chamber of the clutch determines fan speed. The working volume of the silicone oil is controlled with an electrically actuated valve.
In general, a maximum slip power value is determined for a specific fan clutch drive to ensure its proper performance and durability. Given this pre-determined maximum slip power level, calculations based on fan drive input speed values and the torque constant of the fan or fan clutch drive yield a corresponding family of slip power control, fan speed values that define the boundary of an excessive slip power region. Such slip power control, fan speed values may be continually calculated by the fan drive controller for comparison with the current controller determined, desired fan drive speed. Alternatively, and preferably, a family of slip power control, fan speed values for the operating range of fan drive input speed values are pre-calculated and stored in the controller database for retrieval during its control cycles.
The fan drive controller then continually compares its desired fan speed, based on current cooling requirements, with the applicable slip power control, fan speed values. If these comparisons indicate that the desired fan speed will not cause the fan drive to operate in the excessive slip power region no correction is made. But if these comparisons indicate that the desired fan speed will cause the fan drive to operate in the excessive slip power region a correction to the desired fan speed may be made, provided engine cooling is not adversely affected, such that the fan drive will operate at or below the maximum slip power level. While it may be necessary to occasionally and temporarily exceed the pre-determined maximum slip power value to satisfy higher priority engine cooling requirements, the excessive slip power operating region can generally be avoided.
Other objects and advantages of the invention will become apparent from a detailed description of preferred embodiments which follows.